Papers by Author: You Shi Hong

Abstract: Molecular dynamics simulations have show that nanocrystalline (NC) materials can be treated as composite materials consisting of two phases of grain and grain boundary. In this paper, the incremental stress-strain relation is derived based on deformation mechanism of NC materials and internal variable theory from micromechanics point of view. The developed model is exemplified by the pure copper subjected to uniaxial tension. Implicated iteration algorithm is then employed to obtain the stress-strain relation. Moreover, the effects of grain shape and statistical distribution of grain sizes are also discussed, and predicted results are compared with experimental values to verify the model.

Abstract: In this paper, rotary bending fatigue tests for a structural steel were performed in laboratory air, fresh water and 3.5% NaCl aqueous solution, respectively, thus to investigate the influence of environmental media on the fatigue propensity of the steel, especially in high cycle and very-high-cycle fatigue regimes. The results show that the fatigue strength of the steel in water is remarkably degraded compared with the case tested in air, and that the fatigue strength in 3.5% NaCl solution is even lower than that tested in water. The fracture surfaces were examined to reveal fatigue crack initiation and propagation characteristics in air and aqueous environments.

Abstract: Very-High-Cycle Fatigue (VHCF) is the phenomenon of fatigue damage and failure of
metallic materials or structures subjected to 108 cycles of fatigue loading and beyond. This paper
attempts to investigate the VHCF behavior and mechanism of a high strength low alloy steel (main
composition: C-1% and Cr-1.5%; quenched at 1108K and tempered at 453K). The fractography of
fatigue failure was observed by optical microscopy and scanning electron microscopy. The
observations reveal that, for the number of cycles to fatigue failure between 106 and 4×108 cycles,
fatigue cracks almost initiated in the interior of specimen and originated at non-metallic inclusions.
An “optical dark area” (ODA) around initiation site is observed when fatigue initiation from
interior. ODA size increases with the decrease of fatigue stress, and becomes more roundness.
Fracture mechanics analysis gives the stress intensity factor of ODA, which is nearly equivalent to
the corresponding fatigue threshold of the test material. The results indicate that the fatigue life of
specimens with crack origin at the interior of specimen is longer than that with crack origin at
specimen surface. The experimental results and the fatigue mechanism were further analyzed in
terms of fracture mechanics and fracture physics, suggesting that the primary propagation of fatigue
crack within the fish-eye local region is the main characteristics of VHCF.

Abstract: The nanocrystalline (nc) formation was studied in cobalt (a mixture of ε (hexagonal close packed) and γ (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of
{ 10 10 }〈 1120 〉 prismatic and {0001}〈 1120 〉 basal slip in the ε phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the γ phase. The strain-induced γ→ε martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the γ phase was interpreted in terms of the martensitic transformation and twinning.

Abstract: The mechanism of fatigue crack nucleation for nanocrystalline (nc) nickel was
experimentally investigated in this paper. The samples of electrodeposited nc nickel were loaded
cyclically by using a three point bending instrument at first. Then, atomic force microscopy (AFM)
was used to scanning the sample surface after fatigue testing. The results indicated that, after fatigue
testing, there are vortex-like cells with an average size of 108nm appeared along the crack on nc
nickel sample. And, the roughness of sample surface increased with the maximum stress at the
surface

Abstract: In this paper, torsion fracture behavior of drawn pearlitic steel wires with different heat
treatments was investigated. Samples with different heat treatment conditions were subjected to
torsion and tensile tests. The shear strain along the torsion sample after fracture was measured.
Fracture surface of wires was examined by Scanning Electron Microscopy. In addition, the method of
Differential Scanning Calorimetry was used to characterize the thermodynamic process in the heat
treatment. A numerical simulation via finite element method on temperature field evolution for the
wire during heat treatment process was performed. The results show that both strain aging and
recovery process occur in the material within the temperature range between room temperature and
435 °C. It was shown that the ductility measured by the number of twists drops at short heating times
and recovers after further heating in the lead bath of 435 °C. On the other hand, the strength of the
wire increases at short heating times and decreases after further heating. The microstructure
inhomogeneity due to short period of heat treatment, coupled with the gradient characteristics of shear
deformation during torsion, results in localized shear deformation of the wire. In this situation, shear
cracks nucleate between lamella and the wire breaks with low number of twists.